This invention relates to optical position sensors and more particularly relates to encoder plates used in fiber optic position sensors for monitoring displacements of movable bodies. Also, this invention relates to a method of designing an optical position sensor.
U.S. Pat. Nos. 4,275,965, 4,117,460, and 4,116,000 relate to fiber optic position sensors for monitoring displacements of movable bodies. Specifically, U.S. Pat. No. 4,275,965 relates to a high resolution optical sensor, utilizing fiber optic encoder plates for measuring the position of a reciprocating device over a long stroke distance. U.S. Pat. Nos. 4,117,460 and 4,116,000 relate to optical position sensors having conventionally designed encoder plates and utilized in systems for controlling variable engine parameters of a modern gas turbine engine, such as fuel flow, flow-path contour, and pressure along the flow path.
In general, fiber optic position sensors comprise a light source transmitter, a light sensing receiver, an encoder plate (located between the light transmitter and the receiver) for modulating light as a function of position, and a digital switching device triggered by a preset threshhold level of light output sensed at the receiver. The encoder plate comprises several rows of apertures in a plate which is attached to a body, whose position it is desired to monitor, so that the plate moves between the light transmitter and light receiver in response to displacements of the body. Each row of apertures forms an information channel. Typically, the position sensor is designed so that the digital switching device produces several symmetrical digital output signals in response to the light output sensed at the receiver as a function of encoder plate position. Such symmetrical digital output signals may be easily processed by electronic circuitry which is relatively uncomplicated in design and operation and which is relatively inexpensive to construct. Conventionally, in order to produce the desired symmetrical digital output signals, each information channel on the encoder plate is constructed with apertures of equal length which are equally spaced from each other a distance equal to the length of an aperture and the threshhold level of the digital switching device is preset at one-half of the designed (nominal) maximum amplitude of the light output at the receiver.
The symmetry of the digital output signals, and therefore, the accuracy of the fiber optic position sensor, depends on maintaining the light signal amplitude, sensed at the receiver, at the designed operating levels. However, the position sensor is usually located in a hostile operating environment and variations in the amplitude of the light output sensed at the receiver may occur due to varying operating temperatures, degradation in the operating characteristics of the sensor due to aging of the equipment, breakage of optic fibers, and the accumulation of dirt and oil on the encoder plate. These operating conditions cause undersirable variations in the accuracy of the position measurements which are proportional to the amplitude of the light transmissions.